1. Field of the Invention
The present invention relates generally to electric vehicles, and more particularly, to a technique for suppressing a pitching motion of an electric vehicle.
2. Description of the Related Art
Movement of attitude that is determined by expansion and contraction of the suspension mounted on vehicle's front and rear wheels is called the pitching motion, and the attitude changes according to the particular traveling state of the vehicle or the particular state of the road surface. For example, accelerating or decelerating the vehicle generates a pitching moment, the moment about the gravitational point of center, in the direction where the longitudinal direction of the vehicle chassis changes upwardly or downwardly, and the pitching moment causes the pitching motion. If the road surface on which the vehicle is traveling has bumps, the pitching motion will also result from vertical vibration of the front and rear wheels with a difference in time.
In order to suppress such changes in attitude due to the pitching motion, the vehicle has a suspension system that includes springs and dampers for damping the vibration of the wheels.
The vehicle is adjustable in both riding comfort and steering stability by assigning characteristics to the springs and dampers in the suspension system, but it is very difficult to balance riding comfort and steering stability. This is because, since riding comfort and steering stability are contradictory characteristics and since the actual vehicle weight changes significantly with the number of passengers and the quantity of goods loaded, optimal data that allows response to all situations is difficult to determine.
Meanwhile, there is known a technique (refer to JP-62-12305-A, for example) that is intended for complementing the functionality of a suspension by controlling the driving force of the vehicle to stabilize the attitude of the vehicle chassis. The technique described in JP-62-12305-A suppresses the pitching motion of the vehicle by responding appropriately. More specifically, if the front wheel of the vehicle chassis moves upward, the vehicle is reduced in driving torque by utilizing road-surface repulsion to reduce the moment applied in the direction that the front wheel moves upward, and conversely if the front wheel moves downward, the vehicle is increased in driving torque by utilizing road-surface repulsion to increase the moment applied in the direction that the front wheel moves upward.